The auditory system consists of ascending pathways that transmit information from the ear to the cortex, where sound is perceived, and descending pathways from the cortex that modulate the processing and flow of ascending information. The descending pathways play a role in a wide variety of functions, including selective attention, learning, frequency selectivity, sound localization, and discrimination of speech sounds. Much of the previous work on descending pathways has focused on projections from the brainstem to the cochlea. Characterization of the morphology, physiology and transmitter chemistry of the cells that project to the cochlea has led to major advances in our understanding of this olivocochlear system. Our understanding of the remaining descending pathways, some of which involve many more cells than the olivocochlear system, lags far behind. We propose to use combinations of sensitive new anatomical tracing techniques to examine the functional organization of some of the largest components of the descending auditory pathways. Our experiments are focused on the projections from the auditory cortex to the brainstem auditory nuclei. The importance of this pathway was highlighted by recent discoveries that the cortical projections are far more extensive than previously believed, extending as far as the cochlear nucleus, which is the site of entry of auditory information into the brain. One goal of our experiments is to identify the brainstem circuits that are contacted directly by descending projections from the cortex. A second goal is to identify the anatomical relationships between the cells that give rise to the descending projections and the functional organization of auditory cortex. The results should provide insights into the types of information being transmitted by the descending projections as well as the identity of the brainstem circuits that are affected by these projections. This information will provide in turn an anatomical basis for interpreting the results from other types of studies and, eventually, help us understand the role of the descending pathways in both the normal and the diseased or damaged brain.